UMTS is a mobile system of a third generation which will be launched in the year 2002. It is a universal system in the sense that the radio network for UMTS (called UTRAN) is intended to be connected to different kinds of core networks (GSM, NSS, ISDN, IP etc).
GSM and NMT are two examples of existing mobile systems (systems of the 2:nd respective 1:st generation) which have been standardised and optimised to in the fist place offer a speech service which in its nature is a real time service. In addition to the speech service there have during the years been launched different data services as complement. The latter, which are using GSM as carrier, is HSCSD and GPRS. These services utilise a large part of the existing GSM-functionality and infrastructure. Both HSCSD and GPRS have more or less their own tailored functionality.
UMTS is, in contrast to the mobile systems of today, intended to offer a lot of different real-time services and non real-time services (both interactive and non-interactive services) with different demands on Quality of Service. This abundance of services and applications calls for new demands on the future system.
In a mobile telecommunication system, mobile stations are movable and have to be located by the network in case of an incoming call. A frequent type of network systems are cellular systems. In such a system the service area consists of cells, where each cell is served by a base station.
The mobile system allows user mobility, which means that a mobile station can transmit and receive data from just any place within the service area. In order to support user mobility a set of specific procedures are implemented in the network system.
These procedures include Handover (HO), which is used to maintain a link between the mobile station and the network when the mobile station (MS) moves in the cellular surroundings. The HO-procedure is performed in active transmission mode in order to provide continuous service, for instance during a call.
Others of these procedures are about the position management, also called Location Management (LM), with the intention to make clear the location of the mobile station within the service area. LM-procedures are performed in non-transmission mode.
The Location Management strategy, also called the LM-strategy, is a plan for location management of the mobile station, and represents an agreement between the mobile station (MS) and the network, which both must be involved in the location management.
The location management is supported by procedures called “paging”, or search, and “location update”, or updating of location. Paging is a procedure to identify the location of a mobile within a certain paging area. Paging is activated by incoming call and is performed by the network within the paging area.
A paging area consists of one or more cells, the base stations of which transmit the paging message to all mobile stations which are served in these cells, as is shown in FIG. 1. The paging message, which usually is transmitted on the signal channel, loads the system and may cause jamming of radio transmitted information.
Location update is a procedure which is used by the mobile station in order to update the network regarding the location of the mobile station.
The Location Update procedure (LU-procedure) can be initiated by the mobile station, or be forced by the network. The network transmits system information in each cell and, depending on this information, the mobile station can take the initiative to update its location in the network. This is made by a location update message being transmitted from the mobile station to the network, which normally also is made on the signal channel. This procedure is shown in FIG. 2. The location of the mobile station is stored in a special database, a location register, in the network, and the location is represented by a location area. A location area can consist of one or more cells. The location update message is transmitted for instance when the mobile station is moved from one location area to another.
Depending on factors such as the size of the service area, the mobility of the mobile station, and its call frequency, different LM-strategies can be suitable. A number of LM-procedures can be divided into three categories: Area-based procedures, non-area-based procedures, and individual procedures.
An area-based procedure relates to, as is implied by the name, a geographical division of the service area. The easiest way for a network to find a mobile station is to page it over the whole service area. This requires that a paging message is transmitted in each cell in the network. In addition to being the easiest way, it is also the most expensive from a signal point of view. By such a procedure the mobile station need not report its location within the service area. The cost of the paging depends on the size of the service area and the network; the larger service area and network, the more expensive will it be to find the mobile station. This type of procedure is suitable for networks with very limited service areas and with few cells. The procedure also can be usable for network at paging for highly prioritised users. The advantages of such a procedure is that no location register is required, and consequently neither a corresponding database in the network, which will reduce the signal load in the fixed network.
One way to reduce the signal traffic is to divide the service area in smaller parts called paging areas. The network pages the mobile station in one paging area at a time, and when the network only has got a response from the mobile station, the paging procedure is stopped. Even if the introduction of paging areas will reduce the signal traffic, such a procedure will cause not negligible time delay problems in larger networks. By time delay is here meant the period from the transmission of the first paging message until answer has been given from the mobile station. The size of the paging area should be decided with regard to the time delay requirements. Neither for a procedure like this there is any location register required.
In order to reduce the signal load from the network, the service area can be divided into location areas. By the mobile station signalling its location to the network, the network will be kept informed about the movements of the mobile station or current locations. This information is stored in a database. The procedure when the mobile station informs the network about its location is called Location Update (LU). This procedure is initiated and performed by the mobile station based on its geographical location. The mobile station usually listens for messages that the network transmits, and compares current location information with old. When necessary, the mobile station initiates location update, at which information about its new location is given to the network. At incoming call the network knows in which location area the mobile station is, and where it should page. In the case that a location area is the same as a paging area, its size will decide the total signal load for the system. A small location area requires a lot of location updates, and a large location area requires large paging load. Therefore an optimisation of the size of the location area is required, which is made by adaptation of the parameter adjustments of the location area. Performance analyses have shown that an optimal size of a location area depends on the average removal and frequency of incoming calls to the mobile station. This procedure is used in the GSM-system. Such a location management procedure requires a location register in the network.
In order to reduce the paging load, the location area can be divided into smaller paging areas. Such a procedure will reduce the paging load, but will at the same time result in an increase of the time delay.
An alternative, or completing procedure of location management, is to make the mobile station inform the network when it has moved a certain distance from the location which was last reported. Such a procedure can be used in CDMA-based systems (Code Division Multiple Access), such as IS95. In a CDMA-based system, the base stations can be informed about latitude and longitude of the mobile station when it has performed location update. The base station then will become the centre of a circle. The mobile station calculates a distance function based on said latitude and longitude values, and when the mobile station is moved outside the radius of the circle a new location update is performed. That base station that registers the location update will be the new central point. Such a procedure can also be used in other mobile systems by utilising a positioning system like GPS (Global Positioning System).
Procedures which are initiated independent of the geographical surroundings and location areas and paging areas, but yet are general, can be called non area-based procedures. Such procedures can normally be combined with one or more of the area-based procedures described above. This concerns, for instance, registration of whether the mobile station is connected to the network. For that reason the mobile station transmits a location update message each time it is connected or disconnected. This procedure is used in most mobile systems, such as GSM, and prevents to a great extent unnecessary paging from the networks.
Another non-area based procedure for location update is a timer-based procedure. In certain systems, such as GSM, the disconnection procedure is not acknowledged by the network. This means that if a disconnection message has not reached the network, the network does not know it, and in case of an incoming call to the mobile station, the network has to page the mobile station in spite of that it is disconnected. The solution of such a problem can be a time-based location update procedure, which can be called periodic location update. Periodic location update is performed when a timer expires. The mobile station and the network have the same timer values. If the timer expires and no location update is performed, the network will set a flag to disconnected. This procedure is efficient when the battery runs out, or when the mobile station moves to an area which is not covered by the mobile system, and can save a great deal of unnecessary paging. An alternative to timer-based procedures can be network-forced location updates. The network has the possibility to force a location update by transmitting a message to the mobile station and force it to update its location. This procedure can be useful in case when the mobile station during a period of time has been in an area which is not covered by the mobile system, before it returns to the service area.
A third overall group of procedures for location management is based on individual management of the mobile stations. Mobile stations, and their users, differ regarding mobility, frequency of incoming calls, and which applications that are used. A location management procedure with dynamic adjustment of performance parameters therefore is desirable. It is well known that the characteristics of mobile stations varies individually for each mobile station, and varies for an individual mobile station over time and space. Such procedures are memory based and require a database to store the location of the mobile station during a period of time. Algorithms for sophisticated location and estimation of speed can be useful to effect location management procedures.
One way of utilising individual location management procedures is to classify mobile stations based on their mobility, for instance speed, or based on their frequency of incoming calls and the used application. This implies that the location management procedure and its parameter adjustments are adapted to the user's behaviour.
In a case with constant frequency of incoming calls, but with different mobility for different mobile stations, slower mobile stations may require smaller update areas and paging areas than the mobile stations which are moving quicker. A possible solution is to divide mobile stations in two groups with two different sizes of location update areas and paging areas.
Another thing that makes a difference between different mobile stations is the application that is used; it is for instance a big difference between typical speech traffic and data traffic. A voice call is characterised in that a relative constant transmission speed is required, whereas data traffic is more burst like. As is illustrated in FIG. 3, typical data traffic includes periods when no traffic exists. Use of traditional location management in a packet oriented system intended for data transmission can load the system and imply high paging costs. On the other hand data traffic is less sensitive to delay than speech traffic. This problem can be solved by dividing the mobile stations into groups according to their applications. Each group has its own location management strategy.
The location update procedure is always performed in cells at the borders to next location area. Consequently the update traffic of the location requires that extra resources are provided at the bordering cells. A geographical distribution of location update traffic therefore can be of interest. Classification of mobile stations in groups distributes the location management traffic over their cells, where each mobile station or a group and each group has a specific mapping of location areas.
Estimations of optimal performance parameters according to the above can further be developed to include dynamic adjustment of the performance parameters. Especially habitual movements of mobile stations can be utilised for a location management procedure with individual paging areas. By saving the location for each mobile station during a period of time in a special database, the network can calculate the location probability for each mobile station. The mobile station is first paged in the paging areas that have the highest location probability. The probability is estimated on basis of data which have been stored when the mobile station has been in active transmission mode. FIG. 4 illustrates by an example this procedure. X is a person who is living in area A and is working in area B. Area C in between represents the travelling distance from the home area A to the working place area B and back. These three areas A, B, C can be the most probable areas in which to find person X and consequently constitute the highest prioritised paging areas. Depending on which time of the day that an incoming call is coming to X, the paging starts in area A, B, or C. The person Y, on the other hand, is someone who is living and working in the same area D. This area consequently constitutes the highest prioritised paging area, since there is a high probability to find person Y here all over the 24-hour period. The selection of paging areas based on probability of location minimises the paging cost, but may cause problems in time delay sensitive systems.
A third possibility with individual procedures is to utilise a predictive strategy. A lot of mobile stations differ with regard to their mobility. A certain class of mobile stations can have a specific direction with a constant speed over a period of time. At certain points of time, a registration of the location of the mobile station is performed in the network. When this is made, a predictive calculation is performed of the location of the mobile station by the location management database and the mobile station itself. After that, the mobile station regularly compares its current location with the calculated location. If the difference is more than a predefined value, a location update is performed and a new calculation. Based on this predictive location area, the network pages the mobile station. Algorithms for positioning and estimation of speed are usable for a procedure like this which reduces the load as a consequence of paging at the cost of a large need for processor power.
The procedures which have been described above have different advantages and disadvantages. Which location management procedure, based on one of the above mentioned procedures or another procedure, that is best suited for a specific case depends on a plurality of factors. In order to leave as much space as possible in the air interface for the application itself, for instance speech transmission, it will be of greatest interest to minimise the signalling which is made in the location management. Another important, and sometimes contradictory, aspect is the demand for low time delay which can occur as a consequence of paging in a plurality of paging areas. Further, certain advanced procedures may require sophisticated and expensive equipment in the fixed parts of the networks, in form of hardware and software.
In order to optimise the location management, the mobile systems make use of a location management strategy which can be based on a plurality of procedures. The GSM-system, for instance, makes use of a strategy which is suitable in the first place for speech traffic, for which the system principally is created. As has been mentioned above, this strategy includes a procedure which utilises location areas, but also procedures for connection and disconnection and a timer-based procedure. This location management strategy is associated to the carrier, i.e. the GSM-system.
For data transmission there is a patent-oriented system called GPRS (General Packet Radio Service) which uses GSM as carrier. As far as location management is concerned, a procedure, however, has been added which will make GPRS more suitable for data transmission. Mobile stations are divided into two groups depending on their mobility status “Ready” and “Standby”. A mobile station with mobility status “Ready” has a cell as paging and location area, whereas a mobile station with the mobility status “Standby” has a larger area including a plurality of cells as paging and location area. A third status called “Idle” also exists, corresponding to something like non-active transmission mode for GSM. Change of status for a mobile station can be forced or initiated by a timer expiring. All data services which are intended to be used with GPRS as carrier will, according to the standard, make use of the same type of location management strategy, in spite of the fact that these data services among themselves can be very different with regard to quality requirements and traffic behaviour.
Another system for data transmission is HSCSD (High Speed Circuit Switched Data) which also uses GSM as carrier. This system, however, uses the same location management as GSM.